Assessment of Physiology

4.3.1. Electrophysiology

Electrophysiology is another invaluable tool in the analysis of neuronal function. Drosophila was one of the first few genetically malleable organisms amenable to electrophysiology, and was used to study the NMJ as early as the mid-1970s22. The small size of C. elegans neurons initially appeared prohibitive to electrophysiology, but such limitations were overcome in the 1990s23. Spontaneous, pharmacologically stimulated, or nerve-evoked muscle responses can now be recorded in both Drosophila and C. elegans.

4.3.2. Pharmacology

Drugs that impair synaptic transmission are powerful tools for studying C. elegans NMJ function. The ACh esterase inhibitor, aldicarb, can be used to measure the steady-state ACh release in the live worm. Aldicarb causes accumulation of ACh at the NMJ leading to paralysis and death. Mutants that have reduced release of ACh are aldicarb resistant and mutants that have increased release are aldicarb hypersensitive. Aldicarb has been used successfully to analyze and isolate many synaptic transmission mutants24. A second drug, levamisole, is a nematode-specific nicotinic agonist that activates an ACh receptor in muscle25. Acute exposure to high concentrations of levamisole leads to paralysis and death. Worms are resistant to levamisole if levamisole-sensitive ACh receptors or their downstream effectors are defective. Aldicarb and levamisole can be used to determine if specific structural defects lead to detrimental effects on the physiology of neurotransmission.

4.3.3. Calcium Imaging

Neuronal excitation causes transient changes in calcium levels at the synapse, which control neurotransmitter release. These calcium transients can be detected using genetically encoded fluorescent intracellular calcium sensors. These calcium indicators can be targeted to specific cells and structures. A number of sensors have been developed26. The selection of an appropriate indicator and the correct interpretation of the optical signals depend on the sensor's unique properties. It is possible to analyze and compare electrically evoked calcium transients in individual connections made by a single neuron27.

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